Flexible micromachined transducer device and method for fabricating same

ABSTRACT

Techniques and structures for providing flexibility of a micromachined transducer array. In an embodiment, a transducer array includes a plurality of transducer elements each comprising a piezoelectric element and one or more electrodes disposed in or on a support layer. The support layer is bonded to a flexible layer including a polymer material, wherein flexibility of the transducer array results in part from a total thickness of a flexible layer. In another embodiment, flexibility of the transducer array results in part from one or more flexural structures formed therein.

BACKGROUND

1. Technical Field

Embodiments of the invention generally relate to micromachinedtransducer arrays, and more specifically pertain to structures forproviding flexibility of such arrays.

2. Background Art

Transducer devices typically include a membrane capable of vibrating inresponse to a time-varying driving voltage to generate a high frequencypressure wave in a propagation medium (e.g., air, water, or body tissue)in contact with an exposed outer surface of a transducer element. Thishigh frequency pressure wave can propagate into other media. The samepiezoelectric membrane can also receive reflected pressure waves fromthe propagation media and convert the received pressure waves intoelectrical signals. The electrical signals can be processed inconjunction with the driving voltage signals to obtain information onvariations of density or elastic modulus in the propagation media.

Transducer devices can be advantageously fabricated inexpensively toexceedingly high dimensional tolerances using various micromachiningtechniques (e.g., material deposition, lithographic patterning, featureformation by etching, etc.). Such arrayed devices include micromachinedultrasonic transducer (MUT) arrays such as capacitive transducers(cMUTs) or piezoelectric transducers (pMUTs), for example.

Many ultrasound applications—such as intravascular ultrasound (IVUS),endoscopic ultrasound (EUS) or other medical sonography techniques—usecatheters or other such instruments having non-planar surfaces.Typically, transducer arrays are positioned to avoid, or sized toaccommodate, a somewhat small radii of curvature (e.g. ˜5-10 mm) of suchnon-planar surfaces. However, as successive generations of suchinstruments continue to scale in size, there is an attendant push fortransducer arrays to support operation on surfaces having smaller radiiof curvature.

BRIEF DESCRIPTION OF THE DRAWINGS

The various embodiments of the present invention are illustrated by wayof example, and not by way of limitation, in the figures of theaccompanying drawings and in which:

FIGS. 1A through 1D illustrate example configurations of devices thateach include a flexible transducer array according to a respectiveembodiment.

FIGS. 2A through 2P are cross-sectional views which each illustrateelements of a stage of a process for fabricating a flexible transducerarray according to a respective embodiment.

FIG. 3 is a flow diagram illustrating elements of a method to fabricatea flexible transducer array according to an embodiment.

FIG. 4A is a layout diagram illustrating elements of a flexibletransducer array according to an embodiment.

FIG. 4B is a layout diagram illustrating elements of a flexibletransducer array according to an embodiment.

FIG. 5 is a functional block diagram illustrating elements of anultrasonic transducer apparatus that employs a flexible transducer arrayaccording to an embodiment.

DETAILED DESCRIPTION

Embodiments discussed herein variously allow for flexibility of an arrayof piezoelectric transducer elements. For example, techniques andstructures discussed herein variously provide a transducer arraycomprising microelectromechanical system (MEMS) structures, where thetransducer array may be flexed to have a radius of curvature as smallas, or even less than, 1 mm.

Certain techniques and structures discussed herein additionally oralternatively enable electrical interconnects and/or routing which aresuited for applications wherein a transducer array is to operate on acurved surface of a small sensor device. For example, certainembodiments variously provide for a reference voltage (e.g. ground)and/or drive/sense signaling to be communicated through a “back” side ofthe transducer array—i.e. where piezoelectric membrane structures of thearray are configured to transmit/receive ultrasonic (or other) signalsvia an opposite “front” side of the transducer array. By contrast,existing planar (non-flexible) sensors have interconnect contacts ontheir respective front sides.

In some embodiment, piezoelectric membrane structures andinterconnect/signaling structures of the transducer array are integratedtogether in a layered structure which includes a flexible layer ofpolymer material. Interconnects for the transducer elements may extendthrough such a flexible layer to allow for the exchange of a referencevoltage, drive/sense signals and/or the like. For example, transducerelements may variously exchange such voltages/signals via the flexiblelayer with another underlying flexible layer, with an applicationspecific integrated circuit (ASIC) and/or the like.

In certain embodiments, transducer array structure may at some point beformed on and/or coupled to a wafer so that they may be processed at awafer level (e.g. rather than at a die level or other smaller level).Subsequent processing may include cutting transducer array structuresinto one or more individual transducer arrays for respective die-levelapplications. For example, such cutting may form one or more 1 D (e.g.including a 1×n array of transducer elements, for some integer n>1)transducer arrays and/or one or more 2D transducer arrays (e.g.including a m×n array of transducer elements, where integers m, n>1).

FIGS. 1A-1D illustrate example configurations of devices that includeflexible arrays of piezoelectric transducer elements. In someimplementations, a transducer device includes a curved transducer arraycomprising a single row of transducer elements. Some or all of aplurality of elements of the transducer array may be distributed along acurved line. As shown in FIG. 1A, a transducer device 100 a may includea handle portion 104. A transducer array 106 of device 100 a may beattached to handle portion 104 at one distal end 108 where the shape ofthe handle 104 is modified (e.g., widened, flattened, etc.) toaccommodate the shape and size of the transducer array 106. In thisexample, a vibrating surface of the transducer array 106 is along anarc, part of which faces in along a long axis of the handle 104. Inother implementations, the exposed surface of the transducer array 106may instead be along an arc, a part of which faces in a directionperpendicular (or at an acute angle) to the long axis of the handle 104.An operator of the transducer device 100 a may manipulate the handle 104to change the direction and location of the vibrating surface of thetransducer array 106 as desired (e.g., facing the area(s) to be imaged).

The piezoelectric transducer device 100 a may optionally include anintegrated ASIC wafer (not shown) below the array of vibrating elements106 and inside the handle portion 104 (e.g., inside the widened andflattened first distal end 108). Wires 110 connecting to the externalinput connections of the ASIC wafer may exit from the back end of thehandle 104 and be connected to external equipment (e.g., a controldevice and/or a display device).

In some implementations, transducer devices may include two dimensionaltransducer arrays. Each two-dimensional transducer array may includemultiple transducer elements distributed in a curved two-dimensionalarray. The area covered by the two-dimensional array may be of variousshapes, e.g., rectangular, square, circular, octagonal, hexagonal,circular, and so on. The vibrating elements in the two-dimensional arraymay be distributed on a lattice consisting of lines (e.g., a squarelattice or hexagonal lattice) or of more complex patterns. The vibratingsurface of the two-dimensional transducer array may be substantiallywithin a plane, although certain embodiments are not limited in thisregard. The two-dimensional transducer array may be attached to a handle(e.g., at one distal end of a straight cylindrical handle) to form thetransducer device. A plane of a vibrating surface of the transducerarray may include one or more portions which variously face directionswhich are perpendicular to a long axis of the handle or variously facedirections which are substantially parallel to the long axis of thehandle.

An operator of the transducer device may manipulate the handle of atransducer device to change the facing direction and location of thevibrating surface of a two-dimensional transducer array as desired(e.g., facing the area(s) to be imaged). For example, as shown in FIG.1B, a transducer device 100 b includes a side-facing transducer array126 that runs along a circumference of a distal end 128 of the handle124. The transducer device 100 b also includes wires 120 connected to anASIC wafer (not shown) and exiting a back end of the handle 124.

Referring now to FIG. 1C, a transducer device 100 c according to anotherembodiment includes a forward-facing two-dimensional transducer array136. The transducer device 100 c may comprise a handle portion 134,where transducer array 136 extends along a curved surface of a distalend 138 of handle 134. As shown in FIG. 1C, transducer array 136 maycurve in multiple directions along the surface of distal end 138. Thetransducer device 100 c may further comprise wires 170 connected to anASIC wafer (not shown) and exiting a back end of the handle 134.

The configurations of the transducer devices shown in FIGS. 1A-1C aremerely illustrative. Different combinations of the facing direction(e.g., forward-facing, side-facing, or other facing angles) and overallshape (e.g., flat or curved, linear, polygonal, or annular) of thevibrating surface of entire transducer array, the positions of thetransducer array on the handle, and the layout of the vibrating elementson the transducer array are possible in various implementations of thetransducer devices.

Referring now to FIG. 1D, a transducer device 100 d according to anotherembodiment includes an array of transducer elements 150 disposed on aflexible substrate 160. As shown in view 170 and detail view 180 (not toscale) of FIG. 1D, flexible substrate 160 may allow for transducerelements 150 to be readily repositioned relative to one another.Accordingly, transducer device 100 d may serve as at least part of abandage, patch, sheet or other flexible structure which may be laid on,worn by or otherwise applied to some user—e.g. the illustrative patient172. For example, transducer elements 150 may conform to the surface oftissue 182 of patient 172 and propagate pressure waves into (and/orreceive pressure waves from) tissue 182—e.g. via coupling fluid 184(e.g. a gel). In an embodiment, a cable 174 included in or coupled totransducer device 100 d comprises one or more wires which, for example,connect to external input connections of an ASIC wafer and/or connect toother external equipment (e.g., a control device and/or a displaydevice).

In addition, depending on the applications (e.g., the desired operatingfrequencies, imaged area, imaging resolutions, etc.), the total numberof vibrating elements in the transducer array, the size of thetransducer array, and the size and pitch of the vibrating elements inthe transducer array may also vary. In a given array, the respectivepiezoelectric membranes of transducer elements may each include arespective semi-spherical or semi-ellipsoidal dome structures. Apiezoelectric membrane may span a cavity having a cross-sectional widthranging for example, from 25 μm to 250 μm. In some embodiments, an arraymay include transducer elements which are all arranged with respect toone another along a first dimension. For example, a single dimension (1D) array may include 128 transducer elements arranged to have a pitch of150 μm, where each transducer element has a semi-spherical dome spanninga cavity having a 60 μm width. In another example, a 1 D array includes128 transducer elements having a pitch of 350 μm, each element having asemi-spherical dome which spans a cavity having a width between 64 μm to92 μm. In still another example, a 1D phased array may include 96transducer elements arranged to have a pitch of 130 μm, whereinsemi-ellipsoidal domes of the array all have the same first principalaxis diameter along a first dimension and a similar second principalaxis diameter along a second dimension—e.g. the principal axis diametersin a range from 40 μm to 100 μm. Alternatively or in addition, an arraymay include sets of transducer elements variously arranged along twodimensions. In one illustrative embodiment, a two dimension (2D) arraymay include 64×16 transducer elements with area pitch of 120 μm—e.g.where dome structures of the array each span a respective cavity havinga width between 60 μm and 70 μm.

FIGS. 2A-2M show cross-sectional views for various stages of a processto fabricate a flexible transducer array according to an embodiment.FIGS. 2N-2P show respective cross-sectional views each for a flexibletransducer array according to an embodiment. The processing shown inFIGS. 2A-2J may form one or more of transducer arrays 106, 126, 136, forexample.

FIG. 2A illustrates a stage 200 a of fabrication during which a devicewafer 201 is prepared for subsequent processing to form in or on devicewafer 201 various structures of respective transducer elements. Devicewafer 201 may include any of various substrate materials known in theart as amenable to microelectronic/micromachining processing. By way ofillustration and not limitation, device wafer 201 may include a bulkcrystalline semiconductor such as silicon, and/or materials of varioustypes of semiconductor wafers. Although certain embodiments are notlimited in this regard, the illustrative device wafer 201 includes asilicon on insulator (SOI) material.

Certain embodiments are discussed herein with respect to the fabricationof microdome-type transducer elements which each include convexpiezoelectric structures disposed on a support layer. However, suchdiscussion may be extended to additionally or alternatively apply to anyof various other types of transducer elements—e.g. including transducerelements which instead include flat piezoelectric structures and/orconcave piezoelectric structures. For example, as shown in FIG. 2A,fabrication of piezoelectric transducer elements in and/or on devicewafer 201 may include etching or otherwise forming one or moreholes—e.g. including the illustrative holes 202 a, 202 b which, forexample, extend to or through an insulator layer in device wafer 201.

Referring now to FIG. 2B, another wafer 203 may be bonded to devicewafer 201 at stage 200 b. Wafer 203 may provide material to facilitatethe formation of convex structures of transducer elements. For example,wafer 203 may include a semiconductor material such as that of devicewafer 201—e.g. where wafer 203 includes a silicon on insulator (SOI)material.

As shown in stage 200 c of FIG. 2C, grinding, etching (e.g. with KOH),polishing and/or other processing may result in thinning of wafer 203 toa thickness dx which facilitates the formation of bump structures onwafer 201. For example dx may be between 6 microns and 12 microns,although certain embodiments are not limited in this regard.

Referring now to stage 200 d of FIG. 2D, remaining material of wafer 203may be formed into microbumps—e.g. including the illustrative bumpstructures 204 a, 204 b—which extend above a surface 205. Formation ofsuch microbumps may include selectively removing portions of the thinnedwafer 203—e.g. with operations adapted from conventional lithography andetch techniques—and applying a bump reflow processing to other remainingportions of wafer 203.

Subsequent to the formation of such microbumps, any of a variety ofsubtractive and/or additive semiconductor processing techniques (e.g.,including one or more of material deposition, lithographic patterning,feature formation by etching, etc.) may be performed to deposit orotherwise form transducer structures in or on device wafer 201. Forexample, as shown in stage 200 e of FIG. 2E, structures 206 may bevariously formed on surface 205, where bump structures 204 a, 204 bfacilitate the formation of convex portions of structures 206.

An example of some transducer elements which may result from suchprocessing is illustrated in stage 200 f of FIG. 2F. More particularly,a plurality of transducer elements formed by stage 200 f may includetransducer element 214 a and transducer element 214 b. Although certainembodiments are not limited in this regard, one or more other structuresmay be etched or otherwise formed between transducer elements—e.g. tofacilitate a subsequent bending point between transducer elements 214 a,214 b. For example, stage 200 f includes a cavity 216 which is etched indevice wafer 201 (and, in an embodiment, through one or more structuresformed on device wafer 201).

FIG. 2G illustrates a detail view 200 g of a transducer element such asone of those fabricated in the processing for stage 200 f. As depictedin view 200 g, a bottom electrode 236 may be disposed on a thin filmdielectric layer 246, such as silicon dioxide, that can serve as asupport, etch stop during fabrication, electrical insulator, and/ordiffusion barrier. Dielectric layer 246 may be disposed by thermaloxidation, for example. In an embodiment, portions of dielectric layer246 under piezoelectric element 232 are subsequently etched away, inaddition to portions of device wafer 201, during etch processing to forma cavity 230. A piezoelectric element 232 may be subsequently formed onbottom electrode 236.

In the illustrative embodiment, piezoelectric element 232 includes LeadZirconate Titanate (PZT), although any piezoelectric material known inthe art to be amenable to conventional micromachine processing may alsobe utilized, such as, but not limited to polyvinylidene difluoride(PVDF) polymer particles, BaTiO3, single crystal PMN-PT, and aluminumnitride (AlN). Bottom electrode 236 may comprise a thin film layer ofconductive material that is compatible (e.g., thermally stable, has goodadhesion, low stress) with the piezoelectric membrane material, such as,but not limited to, one or more of Au, Pt, Ni, Ir, Sn, etc., alloysthereof (e.g., AuSn, IrTiW, AuTiW, AuNi, etc.), oxides thereof (e.g.,IrO2, NiO2, PtO2, indium tin oxide (ITO), etc.), or composite stacks oftwo or more such materials.

A second dielectric layer 224 (including SiNx or SiOx, for example) maybe disposed —e.g. via plasma-enhanced chemical vapor deposition(PECVD)—over portions of bottom electrode 236 and/or piezoelectricelement 232. Dielectric layer 224 may provide for electrical isolationof bottom electrode 236 and a top electrode 234 to be disposed overdielectric layer 224. As shown, the top electrode 234 may be disposed indirect contact with a top surface of second dielectric layer 224. Inthis illustrative embodiment, the top electrode 234 is employed as thereference (ground) plane to shield the transducer element fromelectro-magnetic interference and the surface electrical charge in theambient environment during operation. As such, the bottom electrode 236may be employed for coupling to a drive/sense signal terminal for thepiezoelectric transducer element. Bottom electrode 236 and/or topelectrode 234 may be disposed, for example, by physical vapor deposition(PVD), atomic layer deposition (ALD), chemical vapor deposition (CVD) orthe like.

In an embodiment, etch, mask and/or other processing may provide forvarious cavities to extend through portions of the transducer element.Such processing may form one or more cavities for positioninginterconnect structures, for example. The interconnect structures mayinclude a reference interconnect 228 which is to provide a referencevoltage (e.g. ground), and another interconnect 238—for brevity,referred to herein simply as a drive/sense (or for brevity, simplydrive) interconnect—which is to provide a drive signal and/or a sensesignal. As used herein “drive signal” refers to a signal which is toactivate a piezoelectric element of a transducer device, and a “sensesignal” refers to a signal generated in response to activation of such apiezoelectric element. A drive/sense interconnect may alternatively bereferred to as a signal interconnect or an active interconnect, forexample.

In the detail view 200 g, respective cavities for reference interconnect228 and drive/sense interconnect 238 each extend from a top surface 226of the transducer element through to device wafer 201. Interconnects228, 238 may be variously be built, for example, of plated copper, andelectrically connect to top electrode 234 and bottom electrode 236,respectively. The width of the openings of such cavities may be ˜6 or 7microns, although certain embodiments are not limited in this regard.Interconnects 228, 238 may comprise respective thin film layers ofconductive material (e.g., deposited by PVD, ALD, CVD, etc.) of anyconventional composition capable of providing a low resistance andamenable to patterning, such as, but not limited to, Au, Al, W, or Cu.In some embodiments, interconnects 228, 238 may be electrically isolatedfrom semiconductor material of the device wafer 201 and/or one or morestructures of the transducer element by an isolation material 220 suchas Al2O3—e.g. where isolation material 220 is deposited by ALD.

In an embodiment, another cavity 230 may be etched or otherwise formedunder the transducer element 232. Cavity 230 may subsequently provide aspanning void to allow for vibration of the transducer element 232.Cavity 230 may be defined at least in part by sidewalls 218 of devicewafer 201 which are opposite one another. The cavity 230 may have awidth (e.g. a diameter, where transducer element 232 is circular) of˜100 microns (e.g. 90-110 microns), although certain embodiments are notlimited in this regard. As discussed herein, a transducer element mayfurther comprise or adjoin one or more flexural structures which allowfor flexibility of a transducer array.

Referring now to FIG. 2H, a stage 200 h of the fabrication process isshown wherein a sub-assembly 240 is bonded to a sacrificial wafer 244via a temporary bonding material 242 such as any of polyimide (HD-3007by Hitachi Chemical DuPont Microsystems), WaferBOND HT-series by BrewerScience, photoresist and SU-8. Sub-assembly 240 may include transducerelements formed in a device wafer—e.g. as shown in stage 200 f.Sacrificial wafer 244 may provide for mechanical support for handlingand/or subsequent processing of sub-assembly 240. For example,sacrificial layer 244 may protect the transducer elements duringinversion of sub-assembly 240 and/or thinning of the device wafer.

By way of illustration and not limitation, the device safer ofsub-assembly 240 may be thinned from a total thickness d1 shown in stage200 h to a comparatively small total thickness d2 of a resultingmodified sub-assembly 241, as shown in stage 200 i of FIG. 2I. Suchthinning may be performed using mechanical grinding, dry chemicaletching (DCE) and/or any of various other methods adapted fromconventional wafer thinning techniques. The total thickness d2 may beequal to or less than 100 microns (for example) and, in particularembodiments, may be equal to or less than 30 microns.

The material of the device wafer in sub-assembly 240 may serve as asupport layer providing mechanical support for transducer elementstherein/thereon. Thinning the support layer to the total thickness d2may allow for flexing in and/or between transducer elements, while thesupport layer provides mechanical support for transducer elements duringsuch flexing.

For example, thinning of the support layer to form sub-assembly 241 mayexpose a side 284 of the support layer. Exposure of side 284 may alsoexpose one or more openings to respective cavities formed in the supportlayer. By way of illustration and not limitation, the thinning of thesupport layer may expose openings 280 a, 280 b each to a respectivespanning void for a respective transducer element. Additionally oralternatively, such thinning may expose an opening 282 to a channelwhich is to serve as a flexural structure, as discussed herein.

In an embodiment, the thinning of the support layer may additionally oralternatively expose one or more interconnects of the transducerelements. For example, the exposed side 284 may include exposed portionsfor various reference interconnects and/or drive/sense interconnects forrespective transducer elements. As discussed herein, additionalprocessing may variously couple some or all of such exposedinterconnects each to a corresponding interconnect of a flexible polymerlayer.

Referring now to stage 200 j of FIG. 2J, additional etching or othersubtractive processing may be performed through the exposed openings 280a, 280 b to remove that remaining material of wafer 203 whichfacilitated the formation of concave structures of the transducerelements. In an embodiment, such subtractive processing exposes anunderside of the bottom electrode 236 of a transducer element.

Referring now to FIG. 2K, a processing stage 200 k is shown wherein asub-assembly 250 including the structures shown in stage 200 i is to beadhered or otherwise bonded to a sub-assembly 252. For example, the side284 of the support layer of sub-assembly 241 may be bonded tosub-assembly 252 via an adhesive 258 which includes conductive fillers260. The conductive fillers 260, which may be distributed throughoutadhesive material 258, may provide a conductive path between a pair ofinterconnects which are aligned with one another, and adhered byadhesive material 258. However, the adhesive material between fillers260 may electrically isolate different pairs of such alignedinterconnects from one another. In one illustrative embodiment, theadhesive material 258 comprises benzocyclobuten (BCB) and metal (and/ormetal coated) spheres. Alternatively, a polyimide material such asHD-7000 by DuPont may be used instead of BCB for adhesive material 258.

Sub-assembly 252 may include a flexible supporting structure and variouselectrical connection structures for the transducer elements. In anembodiment, sub-assembly 252 is fabricated on a separate wafer prior toadhesion to sub-assembly 250—e.g. where some or all of the electricalconnection structures are formed variously formed in or on the flexiblesupportive carrier by subtractive and/or additive processing. By way ofillustration and not limitation, sub-assembly 250 may include structuresbonded to or otherwise formed on a sacrificial layer 254 which is toprovide mechanical support for other structures sub-assembly 250 duringsubsequent processing such as the bonding during stage 200 k.

In an embodiment, the flexible supporting structure provided bysub-assembly 252 includes a polymer layer 256 comprising a cured polymermaterial such as polyimide. Examples of other suitable polymer materialsfor polymer layer 256 include polyester (PET), polyethylene napthalate(PEN), Polyetherimide (PEI), along with various fluropolymers (FEP) andcopolymers Polyimide films. Polymer layer 256 may be coated as a singlelayer or multiple layers of polymer material(s).

Electrical connection structures of sub-assembly 252 may comprise one ormore interconnects each corresponding to a respective interconnect inthe support layer of sub-assembly 250. For example, sub-assembly 252 mayinclude interconnects which each extend through the polymer material oflayer 256. Various subtractive and/or additive semiconductor processingtechniques (e.g., including one or more of material deposition,lithographic patterning, feature formation by etching, etc.) may beadapted, according to various embodiments, to variously form suchinterconnect structures in the polymer material. By way of illustrationand not limitation, such interconnects may comprise ground interconnects266 a, 266 b each to couple to a corresponding ground interconnect of arespective transducer element of sub-assembly 250. In an embodiment,sub-assembly 252 further comprises ground contacts 262 a 262 b which areplated or otherwise disposed on a surface of polymer layer 256. Groundinterconnects 266 a, 266 b may variously provide for electricalconnectivity between ground contacts 262 a 262 b, respectively, and thetransducer elements of sub-assembly 250.

Alternatively or in addition, such interconnects may comprisedrive/sense interconnects 268 a, 268 b each to couple to a correspondingdrive/sense interconnect of a respective transducer element ofsub-assembly 250. Sub-assembly 252 may comprise drive/sense contacts 264a, 264 b which are plated or otherwise disposed on polymer layer256—e.g. where drive/sense interconnects 268 a, 268 b variously provideelectrical connectivity between drive/sense contacts 264 a 264 b,respectively, and transducer elements of sub-assembly 250.

Referring now to FIG. 2L, a stage 2001 of processing is shown whereinsub-assemblies 250, 252 are bonded together. After stage 2001,subsequent processing may be performed to remove the sacrificial layer244 of sub-assembly 250 and/or the sacrificial layer 254 of sub-assembly252. A resulting assembly for a processing stage 200 m is shown in FIG.2M. In stage 200 m, a transducer array includes transducers A1, B1,where a flexural structure 270 provides flexibility between transducersA1, B1. Such flexibility may be additionally or alternativelyfacilitated the total thickness d2 of the support layer for thetransducers A1, B2 and/or by the polymer layer underlying transducersA1, B2. For example, FIG. 2N show a stage 200 n for a fabricationprocess according to an alternative embodiment. In stage 200 n, asimilar transducer array including transducers A2, B2 is formed, whereinno structure such as flexural structure 270 is located betweentransducers A2, B2. Flexibility of the transducer array of FIG. 2N mayresult at least in part from a sufficiently thin (e.g. less than orequal to 30 microns) support layer for transducers A2, B2 and a flexiblepolymer layer underlying the support layer.

In another embodiment, adhesion between sub-assembly 250 andsub-assembly 252 may be achieved by low temperature eutectic bonding,such as that achieved with Au and AuSn. By way of illustration and notlimitation, an Au layer may be deposited on portions of sub-assembly 250which are to bond to sub-assembly 252, and an AuSn layer may bedeposited on portions of sub-assembly 252. Prior to eutectic bonding,one or both such layers may be patterned using photolithography andetching to form desirable bonding surfaces for transducers A3, B3—e.g.where resulting bonding points 280 are variously patterned to seal offopenings, provide respective conductive paths for interconnects and/orotherwise provide strong mechanical bonding.

In certain embodiments, sub-assembly 250 and sub-assembly 252 are bondedtogether using a non-conductive paste (NCP) such as one including BCB orpolyimide as a bonding material. Galvanic contacts between metalinterconnects on sub-assembly 250 and respective metal contacts onsub-assembly 252 may be achieved at least in part by conductivestructures of sub-assembly 252—such as interconnects 266 a, 266 b, 268a, 268 b—being etched or otherwise formed to variously extend from suchmetal contacts and form protrusions which extend above polymer layer 256at a height that is at least equal to the adhesion bond-line thickness.

In certain embodiments, the flexible layer of a transducer arrayincludes one or more layers of electrical distribution structures which,for example, provide for operation of and/or signal communication withtransducer elements. For example, FIG. 2P show a stage 200 p of afabrication process which, for example, includes some or all of thefeatures of the process illustrated in FIGS. 2A-2M. In stage 200 p, atransducer array including transducers A4, B4 is similarly formed,wherein a flexible layer for transducers A4, B4 includes conductiveportions 290 which variously extend from interconnect structures suchground interconnects 266 a, 266 b and/or drive/sense interconnects 268a, 268 b. In an embodiment, such conductive portions 290 variouslyextend laterally within the flexible layer, and additionally oralternatively couple to respective contacts on an underside of theflexible layer, to variously distribute power, ground and/or signalingto, from and/or among transducers.

FIG. 3 illustrates elements of a method 300 for fabricating a flexibletransducer array according to an embodiment. Method 300 may includemicromachining and/or other fabrication operations to variously form,for example, some or all of the structures shown in FIGS. 2A-2J.

In an embodiment, method 300 includes, at 310, receiving a support layersuch as device wafer 210. Method 300 may further comprise, at 320,forming a plurality of transducer elements which, for example, eachcomprise a respective piezoelectric element and respective electrodesand interconnects. In an embodiment, the forming at 320 includes, foreach of a plurality of transducer elements, forming on a first side ofthe support layer an electrode of the transducer element and apiezoelectric element of the transducer element, and further forming inthe support layer an interconnect of the transducer element, wherein theinterconnect is coupled to the electrode of the transducer element. Byway of illustration and not limitation, for each of the plurality oftransducer elements, forming the transducer element may include formingon the first side of the support layer a reference electrode and adrive/sense electrode, and forming in the support layer a referenceinterconnect and a drive/sense interconnect. As illustrated in FIGS. 2F,2G, for example, a reference interconnect and a drive/sense interconnectof a transducer element may couple, respectively, to a referenceelectrode and a drive/sense electrode of that transducer element.

Method 300 may further comprise, at 330, thinning the support layer—e.g.after forming the plurality of transducer elements at 320—to expose asecond side of the support layer. The support layer may thinned at 330to a total thickness equal to or less than 35 microns, for example. Insome embodiments, the support layer is thinned at 330 to a totalthickness equal to or less than 30 microns.

In an embodiment, thinning the support layer at 330 includes, for eachof the plurality of transducer elements formed at 320, exposing arespective opening for a spanning void which extends through the supportlayer. In an embodiment, the respective piezoelectric elements of theplurality of transducer elements each span a distance between respectivesidewalls of the support layer, the distance comprising a spanning void.Alternatively or in addition, respective interconnects of the pluralityof transducer elements may each extend through the support layer betweenthe first side of the support layer and the second side of the supportlayer. For example, thinning the support layer at 330 may includeexposing respective reference interconnects and/or drive/senseinterconnects of the plurality of transducer elements.

Method 300 further comprises, at 340, bonding the exposed second side ofthe support layer to a flexible layer. In an embodiment, the flexiblelayer comprises a polymer material and, for each of respectiveinterconnects of the plurality of transducer elements formed at 320, acorresponding interconnect extending through the flexible layer. Thepolymer material may include polyimide, for example. The support layerand transducer elements may be initially formed on a different waferthan one on which the flexible layer is initially formed. In anembodiment, bonding the second side to the flexible layer at 340 mayinclude adhering with an adhesive including conductive fillers. Inanother embodiment, the bonding at 340 may be with a eutectic bond.

Although certain embodiments are not limited in this regard, method 300may comprise forming one or more flexural structures between a firsttransducer element and a second transducer element. Such a flexuralstructure may separate portions of the support layer from oneanother—e.g. where the flexural structure includes a channel whichextends through the support layer between the first and second sides ofthe support layer. In an embodiment, a plurality of flexural structuresmay be formed each between a respective pair of the plurality oftransducer elements. For example, the plurality of flexural structuresmay include channels each formed between respective rows of transducerelements. In an embodiment, such a plurality of flexural structures mayinclude a first channel through the support layer and a second channelthrough the support layer, wherein the first channel and the secondchannel extend along respective lines of direction which intersect oneanother.

FIG. 4A illustrates elements of a flexible 2D pMUT array 400 accordingto an embodiment. Array 400 may include some or all of the features ofone of the transducer arrays shown in FIGS. 2M-2P, for example.

In an embodiment, array 400 may comprise a plurality of transducerelements, as represented by the illustrative twenty (20) transducerelements P11-P15, P21-P25, P31-P35, P41 -P45 and P41-45. The transducerelements of array 400 may be distributed in rows and columns, asrepresented by the illustrative rows R1-R5 along an x-dimension andcolumns C1-C4 along a y-dimension.

One or more channels—e.g. including the illustrative channels G1, G2,G3—may be formed in (e.g. through) the support layer of array 400. Sucha channel may serve as a flexural structure to allow for flexibility ofarray 400. For example, such a flexural structure may facilitateflexibility for the transducer array 400 to conform to a non-planarsurface. As a result, such a transducer array may be fit onto a curvedsensor surface, for example. The channels G1, G2, G3 may function asbreaking lines in the support layer, where the array is mechanicallysupported at the breaking line by the underlying polymer (e.g.polyimide) layer. In an embodiment, an additional polymer layer (notshown) may be built on the back of the first polymer layer—e.g. theadditional polymer layer including conductive connections for directingreference potential and/or signal lines to other test equipmentcircuitry.

For example, array 400 may include sets of signal lines D1-D4—e.g.including drive/sense signal lines and/or reference voltage lines—whichvariously extend under the flexible layer and couple to respectiveinterconnect contacts disposed on a surface of the flexible layer.Signal lines D1-D4 are shown as variously extending along respectiveones of columns C1-C4, although certain embodiments are not limited inthis regard. In an embodiment, the channels G1, G2, G3 and signal linesD1-D4 are on opposite sides (e.g. front and back) of array 400.

FIG. 4B is a plan view of a multi-mode MUT array 450, in accordance withan embodiment. The array 450 includes a plurality of electrode rails460, 470, 480, 490 disposed over an area defined by a first dimension, xand a second dimension y, of a substrate 450. Each of the drive/senseelectrode rails (e.g., 460) is electrically addressable independentlyfrom any other drive/sense electrode rails (e.g., 470 or 480) and are,functionally, separate channels of the array 450. One or morechannels—e.g. including the illustrative channels GX, GY, GZ—may beformed in (e.g. through) a support layer of array 405 between variouslyrespective ones such electrode rails.

Each channel has a characteristic frequency response that is a compositeof the responses from individual transducer elements within the channel.A drive/sense electrode for each channel is electrically coupled inparallel to each element. For example in FIG. 4B, transducer elements461A, 462A, 463A, etc. are coupled together to be electrically driven bythe drive/sense electrode rail 460 (e.g. via respective structurescorresponding functionally to bottom electrode 236). Similarly, alltransducer elements (e.g. including 471A) of another channel are allcoupled together in parallel to the drive/sense electrode rail 470.Generally, any number of transducer elements may grouped together withina channel—e.g. as a function of membrane diameters, element pitches andsubstrate area allocated for each channel.

In an embodiment, at least one membrane dimension varies across elementsof a same channel of the apparatus. Such variation within a channeland/or between channels may provide for ultra wide-band operationalcharacteristics of array 405. As illustrated in FIG. 4B, the circularmembrane diameters vary along at least one dimension of the substrate(e.g., y-dimension) such that each channel includes a range of membranesizes. In the depicted embodiment of array 450, each channel includesthe same number of membranes of a particular size and a same number ofdifferent sizes. As resonance frequency is a function of membrane size(with a higher frequency associated with smaller membrane size), when agiven electrical drive signal is applied to a channel, a particularfrequency response is induced, or when a given frequency response isreturned through a media, it generates a particular electrical sensesignal. For the embodiment depicted in FIG. 4B, where each channel hasthe same population of elements (same number and size distribution), anda same spatial layout, each channel can be expected to have very nearlythe same frequency response. Alternatively, channels with differingelement populations (i.e., a different number of membrane sizes,different number of membranes of a particular size, or different spatialarrangements over the substrate) can be expected to have significantlydifferent frequency responses.

As depicted in FIG. 4B, transducer element 111A a first size (e.g.,smallest diameter membrane) is adjacent to element 112A of a second size(e.g., next larger diameter membrane) with the membrane size graduallyincreasing in a step-wise manner through a first series of elements withever increasing membrane size and then a second series with stepwisedecreasing size back to the smallest diameter. FIG. 4B provides thateach element with some channel population is adjacent to another elementof the same size or of a next smallest or next largest size for anynumber of different membrane sizes (e.g., three, four, or five differentsizes depicted in FIG. 4B, etc.).

FIG. 5 is a functional block diagram of an ultrasonic transducerapparatus 500 that employs a flexible transducer array, in accordancewith an embodiment. In an exemplary embodiment, the ultrasonictransducer apparatus 500 is for generating and sensing pressure waves ina medium, such as water, tissue matter, etc. The ultrasonic transducerapparatus 500 has many applications in which imaging of internalstructural variations within a medium or multiple media is of interest,such as in medical diagnostics, product defect detection, etc. Theapparatus 500 includes at least one transducer array 516, which may beany of the flexible transducer arrays described elsewhere herein. Inexemplary embodiment, the transducer array 516 is housed in a handleportion 514 which may be manipulated by machine or by a user of theapparatus 500 to change the facing direction and location of the outersurface of the transducer array 516 as desired (e.g., facing the area(s)to be imaged). Electrical connector 520 electrically couples drive/senseelectrodes of the transducer array 516 to a communication interfaceexternal to the handle portion 514.

In embodiments, the apparatus 500 includes at least one signalgenerator, which may be any known in the art for such purposes, coupledto the transducer array 516, for example by way of electrical connector520. The signal generator is to provide an electrical drive signal onvarious drive/sense electrodes. In an embodiment, each signal generatorincludes a de-serializer 504 to de-serialize control signals that arethen de-multiplexed by demux 506. The exemplary signal generate furtherincludes a digital-to-analog converter (DAC) 508 to convert the digitalcontrol signals into driving voltage signals for the individualtransducer element channels in the transducer array 516. Respective timedelays can be added to the individual drive voltage signal by aprogrammable time-delay controller 510 to beam steer, create the desiredbeam shape, focus, and direction, etc. Coupled between the pMUT channelconnector 502 and the signal generator is a switch network 512 to switchthe transducer array 516 between drive and sense modes.

In embodiments, the apparatus 500 includes at least one signal receiver,which may be any known in the art for such purposes, coupled to thetransducer array 516, for example by way of electrical connector 520.The signal receiver(s) is to collect an electrical response signal fromeach the drive/sense electrode channels in the transducer array 516. Inone exemplary embodiment of a signal receiver, an analog to digitalconverter (ADC) 514 is to receive voltages signals and convert them todigital signals. The digital signals may then be stored to a memory (notdepicted) or first passed to a signal processor. An exemplary signalprocessor includes a data compression unit 526 to compress the digitalsignals. A multiplexer 518 and a serializer 528 may further process thereceived signals before relaying them to a memory, other storage, or adownstream processor, such as an image processor that is to generate agraphical display based on the received signals.

Techniques and architectures for providing a flexible transducer arrayare described herein. In the above description, for purposes ofexplanation, numerous specific details are set forth in order to providea thorough understanding of certain embodiments. It will be apparent,however, to one skilled in the art that certain embodiments can bepracticed without these specific details. In other instances, structuresand devices are shown in block diagram form in order to avoid obscuringthe description.

Reference in the specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the invention. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment.

Some portions of the detailed description herein are presented in termsof algorithms and symbolic representations of operations on data bitswithin a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the computingarts to most effectively convey the substance of their work to othersskilled in the art. An algorithm is here, and generally, conceived to bea self-consistent sequence of steps leading to a desired result. Thesteps are those requiring physical manipulations of physical quantities.Usually, though not necessarily, these quantities take the form ofelectrical or magnetic signals capable of being stored, transferred,combined, compared, and otherwise manipulated. It has proven convenientat times, principally for reasons of common usage, to refer to thesesignals as bits, values, elements, symbols, characters, terms, numbers,or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the discussion herein, itis appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

Certain embodiments also relate to apparatus for performing theoperations herein. This apparatus may be specially constructed for therequired purposes, or it may comprise a general purpose computerselectively activated or reconfigured by a computer program stored inthe computer. Such a computer program may be stored in a computerreadable storage medium, such as, but is not limited to, any type ofdisk including floppy disks, optical disks, CD-ROMs, andmagnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs) such as dynamic RAM (DRAM), EPROMs, EEPROMs, magnetic oroptical cards, or any type of media suitable for storing electronicinstructions, and coupled to a computer system bus.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the required method steps. The required structurefor a variety of these systems will appear from the description herein.In addition, certain embodiments are not described with reference to anyparticular programming language. It will be appreciated that a varietyof programming languages may be used to implement the teachings of suchembodiments as described herein.

Besides what is described herein, various modifications may be made tothe disclosed embodiments and implementations thereof without departingfrom their scope. Therefore, the illustrations and examples hereinshould be construed in an illustrative, and not a restrictive sense. Thescope of the invention should be measured solely by reference to theclaims that follow.

What is claimed is:
 1. A micromachined transducer array comprising: aplurality of transducer elements each comprising: a respective electrodedisposed on a first side of a support layer including a bulksemiconductor material; a respective piezoelectric element disposed onthe first side of the support layer; and a respective interconnect inthe support layer coupled to the respective electrode of the transducerelement, wherein the interconnect extends through the support layerbetween the first side and a second side of the support layer; and aflexible layer bonded by an adhesive material to the second side of thesupport layer, wherein the flexible layer includes a polymer materialand, for each of the respective interconnects of the plurality oftransducer elements, a corresponding interconnect extending through theflexible layer; wherein the adhesive material provides at least partialelectrical isolation of the respective interconnects of the plurality oftransducer elements from one another; wherein conductive fillers aredisposed in the adhesive material; and wherein the respectiveinterconnects of the plurality of transducer elements are eachelectrically coupled via the conductive fillers to the correspondinginterconnect extending through the flexible layer.
 2. The micromachinedtransducer array of claim 1, wherein a total thickness of the supportlayer between the first side and the second side is equal to or lessthan 100 microns.
 3. The micromachined transducer array of claim 1,wherein the polymer material includes polyimide.
 4. The micromachinedtransducer array of claim 1, wherein each of the plurality of transducerelements includes: a reference electrode disposed on the first side ofthe support layer; a drive/sense electrode disposed on the first side ofthe support layer; a reference interconnect in the support layer, thereference interconnect coupled to the reference electrode of thetransducer element; and a drive/sense interconnect in the support layer,the drive/sense interconnect coupled to the drive/sense electrode of thetransducer element.
 5. The micromachined transducer array of claim 1,wherein the support layer further comprises a flexural structure betweena first transducer element and a second transducer element.
 6. Themicromachined transducer array of claim 5, wherein the flexuralstructure separates portions of the support layer from one another. 7.The micromachined transducer array of claim 1, wherein the flexiblelayer is bonded to the second side of the support layer with a eutecticbond.
 8. The micromachined transducer array of claim 1, wherein theflexible layer includes a first interconnect comprising a first portionto carry a signal laterally along the flexible layer.
 9. An apparatusfor generating and sensing pressure waves in a medium, the apparatuscomprising: the micromachined transducer array of any of claims 1-4 and5; generating means coupled to the micromachined transducer array toapply an electrical drive signal on at least one drive/sense electrode;receiving means coupled to the micromachined transducer array to receivean electrical response signal from at least one drive/sense electrode;and signal processing means coupled to the receiving means to processelectrical response signals received from the plurality of thedrive/sense electrodes.
 10. The apparatus of claim 9, wherein thegenerating means is to apply an electrical drive signal to cause atleast one piezoelectric element of the plurality of transducer elementsto resonate at an ultrasonic frequency.